Do not rearrange header

[ScreamingDev]

This is a header I recently had:

# Copyright (C) 2015 the WordPress team
# This file is distributed under the GNU General Public License v2 or later.
msgid ""
msgstr ""
"Project-Id-Version: Twenty Sixteen 0.1.20150828\n"
"Report-Msgid-Bugs-To: https://wordpress.org/support/theme/twentysixteen\n"
"POT-Creation-Date: 2015-11-20 12:58:54+00:00\n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"PO-Revision-Date: 2015-MO-DA HO:MI+ZONE\n"
"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
"Language-Team: LANGUAGE <LL@li.org>\n"

Parsing it, merging some data with MERGE_ADD and then storing the file again rearranges the header:

msgid ""
msgstr ""
"Project-Id-Version: Twenty Sixteen 0.1.20150828\n"
"Report-Msgid-Bugs-To: https://wordpress.org/support/theme/twentysixteen\n"
"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
"Language-Team: LANGUAGE <LL@li.org>\n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"POT-Creation-Date: 2015-11-20 12:58:54+00:00\n"
"PO-Revision-Date: 2016-06-12T23:05:19+00:00\n"
"Language: \n"

In addition the comment and the Language field is gone.

This shouldn't be when you merge two translations without the MERGE_HEADER option.
It neither should be when you use the option.

[oscarotero]

The automatic alphabetic sorting is added to keep the consistency on generate po files with the same headers but in different order. This eases testing because allows to compare two po output independently of the order in which the headers have been added/modified. This is not done on merge translations, but on generate .po output.
I don't see an issue here, because the headers order does not affect to the content.

[oscarotero]

Included an option to enable the automatic headers sorting (disabled by default)